Various types of implantable medical devices, often referred to as implantable infusion pumps, are used for dispensing controlled volumes of a drug within a patient's body. These devices generally have reservoirs which are filled with a drug for dispensation at a dosage over time that is pre-programmed or programmable. Over time, as the drug is dispensed, the reservoir volume becomes depleted and needs to be refilled. Very often, the reservoir for receiving the drug has a refill port that is sealed with a self-sealing septum. To refill the reservoir, a doctor determines the location of the refill port by palpation of the patient's skin as the refill port typically protrudes from the infusion pump. The doctor the inserts a hypodermic needle through the skin and through the septum into the refill port. Once the needle is within the refill port, the medication is dispensed from a syringe into the refill port where it can refill the reservoir.
It is important to the performance of this process that the needle tip be properly positioned at the desired dispensing location. If the needle is not properly positioned within the refill port, medication can be improperly dispensed into the patient's body. The syringe can include, for example, a full month's worth of medication. It should be self evident that releasing such an amount of medication into the patient is likely to be detrimental.
It is also important to the performance of the refill process that pressures generated as a result of the refill not exceed the capability of the implantable infusion pump to contain the drug. For example, pressure generated in the delivery syringe, and thus at the end of the needle, can easily reach 5 to 10 bars. Such a high pressure level can potentially damage the delivery mechanism of the drug delivery device, often constituting a miniature valve, leading to a release of drug into the patient during the high-pressure episode.
One example of a prior art approach to determining whether a refill needle is properly placed is U.S. Pat. No. 5,171,288 to MacDonald. This patent describes a mechanism that detects the position of a needle based on a resonant circuit that is in an open-circuit state when the needle is not inserted into the refill port. The resonant circuit is closed when the needle is inserted into the port by a flow of electrical charge through the needle, the contact between the needle and the medical device, and body tissues that close the loop from the medical device back to the needle. Problems with this approach include the necessity of transmitting energy even when the needle is not in the port, the need to position an external unit on top of the medical device during refill (perhaps in sterile conditions), and uncertain electrical connections through the patient's tissue.